Method and apparatus for preventing back-fires in metal vapor rectifiers



Feb. 16, 1932. JONAS ET AL 1,845,841

METHOD AND APPARATUS FOR PREVENTING BACK FIRE IN METAL VAPOR REGTIFIERSFiled May 29, 1926 a b v Patented Feb. 16, 1932 UNITED STATES PATENTOFFICE JULIUS JONAS, OF BADEN, AND ERVI'IN KERN, OF VIETTINGEN, NEARBADEN, SWITZER- LAND, ASSIGNORS TO AKTIENGESELLSCI-IAFT BROWN BOVERI &CIE., OF BADEN, SWITZERLAND, A JOINT-STOCK COMPANY OF SWITZERLAND METHODAND APPARATUS FOR PREVENTING BACK-FIRES IN METAL VAPOR RECTIFIERSApplication filed May 29, 1926, Serial No. 112,665, and in Germany June2, 1925.

This invention relates to the prevention of certain undesirablephenomena known as back-iircs in apparatus for rectifying alternatingcurrent of the metallic vapor type, of which mercury vapor rectifiersare an example.

The general object of the invention is the prevention or elimination ofsuch phenomena in a simple and reliable manner.

' A particular object is the provision of a method and apparatus for thepurpose stated, and which produces the desired preventive effect at suchtimes when the tendency for the undesirable phenomena to take place isstrongest.

Another object is the provision of a method and apparatus for thepurpose stated which does not introduce parts into the interior of therectifier which might themselves be the source of undesirable phenomena.

Other and further objects will be pointed out or indicated hereinafter,or be obvious to one skilled in the art upon an understanding of theinvention.

In the drawings forming a part of this specification we show variousarrangements of. apparatus, the same being presented for the purpose ofillustrating the invention. These are not to be construed in any fashionas having the efl'ect of limiting the claims short of the true and mostcomprehensive scope of the invention in the art.

In the drawings,

Fig. 1 is a diagram of apparatus illustrating the invention,

Fig. 2 is a curve diagram illustrating the variation of the anode andshield potentials using the above apparatus,

Fig. 3 is a diagram of another form of ap paratus illustrating theinvention, and

Fig. 4: is a curve diagram showing the variation of anode and shieldpotentials using the apparatus of Fig. 3.

The operation of metal vapor rectificers is liable to be interrupted bythe occurence of baclefires (really internal short-circuits) which areusually accompanied by a number of other harmful effects. Thus aback-fire which ends in a dead short-circuit may have such a destructiveeffect on the material of the rectifier as to necessitate a completeshutdown. When a back-fire takes place, the current flows from theoriginal cathode (or from one of the anodes) to an anode which hastemporarily assumed the functions of a cathode, and it is thereforeclear that a backfire will usually tend to start during the time whenthe potential of the anode concerned is negative to that of the cathode.This condition is fulfilled during that portion of the cycle when theanode is not carrying a load current.

As a general rule the potential taken up by an insulated anode shieldlies somewhere between the potentials of its associated anode and thecathode respectively. If an arc is struck from the anode, however, thisforms a conducting path between the anode and its shield and thecorresponding potentials will then be approximately equal.

Attempts have been made to suppress the back-fires by subdividing theanode shields or giving them certain shapes, but without success, thereason being that the mere presence of the shields cannot prevent thecurrent from passing from cathode to anode. If a tendency to back-fireexists, the increasing glow-discharge current causes the anode shieldsto become positive with regard to the anodes so that the flow of currenttowards them is assisted.

The object of the invention is therefore a means for preventingback-fires in metal vapor rectifiers having insulated anodes, accordingto which insulated metal structures arranged in the path of theback-fire discharge (these may be the anode shields themselves) aremaintained at a potential negative to their associated anode over theperiod within each A. C. cycle during which lib the said anode is notcarrying load current. The potential of the metal structures referred toshall be applied in such a way that the anode is always positive to themetal struc- 5 ture or shields during the'period of maximum tendency toback-fire.

Fig. 2 shows the variation of anode potential during a complete A. C.cycle. If the rectifier has six anodes for example, then 10 each carriesthe load current for one sixth of a cycle. This period is shown by t,and in Fig. 2. During this period the anode is positive with regard tothe cathode. The horizontal line 11 represents the constant potential ofthe cathode. It will be seen that outside the period t t the anode isnegative to the cathode and hence there will be tendency to back-fire.If, however, the anode shield is maintained negative to the 20 anodeduring the danger period the shield will prevent an arc passing from thecathode to the anode, since the signs of the potentials in anode andshield respectively allow the passage of current in one direction only,i. e.

from anode to shield. The curve 6 in Fig. 2 shows the variation in thepotential of the anode shield which should be obtained to secure theeffect of the invention. In the example given it occupies a dehnlteposition relative to the curve a, and may be arrived at by subtracting aconstant amount 0 from the ordinates of a. The anode shleld poten-.

tial may be made to vary in the desired way by the following means :T heshielos are all, 3 connected to a speclal star-connected sec- 'ondarywinding of the transformer feeding the rectifier. This special secondarymay be called an auxihary secondary to distinguish it from the mamwinding. The aux- Qiliary secondary may, for example, be wound withregard to their respective anodes, a relativelv low uni-directionalpotential drop is applied between the respective neutral points of themain and auxiliary secondaries, the sign of this potential drop beingsuch that i -tl1e neutral point of the main winding will be positive tothe neutral point of the auxiliary winding. The necessary potential dropmay be obtained from a small motor generator, rectifier, or battery, andit should be noted that only very small currents will be required.

This embodiment of the invention is illus trated inFig. 1.

The three-phase mains N supply the pri 69 mary P of the transformer Tfeeding rectitier G, the transformer including a main secondary Qconnected as shown to the rectifier anodes (l -(1 and an auxiliarysecondary connected as shown to anode shields in -7a,.

As will be seen from Fig. l, the neutral point 0 of winding Q is joinedto the neutral point 0 of winding Q through the battery B, 0 beingpositive to 0 The negative wire of the D. C. system N starts from therectifier cathode 0 and the positive from K. In the example illustratedthe anode shields h to 72. are insulated. If the voltage at theterminals of winding Q is the same as that of winding Q then thepotential of a shield ill always be lower than that of the asso ciatedanode by an amount 0. Thus a back-fire cannot start. On referring to theFigure 1 it will be seen that resistances w, to 70 are inserted in theleads joining the anode shields with the terminals of the auxiliarywinding Q. These resistances may be ohmic, inductive, or capacitive, andtheir function may thus be explained :The introduction of the potentialdrop 6 undoubtedly fixes the potential of the shields lower than that ofthe respective anodes. The shields however are bombarded by ions comingfrom the cathode and thus there will always be a reverse current flowingthrough the shields, outside the period t t which raises the potentialof the same and may in certain circumstances neutralize the effect ofthe impressed voltage (1. By inserting resistances as w to 10,-, thisreverse current can be reduced to a negligible value.

One disadvantage of the arrangement described is the necessity for asupply of direct current. Suitable low voltage supplies are not alwaysavailable, and in any case complicate the plant. The necessity for a D.C. supply may be entirely avoided. however, by making the voltage of theauxiliary secondary greater than that of the main secondary and notconnecting the neutral points. It appears at first that the potential ofthe auxiliary winding and consequently that of the connected shieldswill still be indefinite, but

as soon as an arc is struck between anode and cathode through theshield, the latter will take up the potential of the anode then carryingload current. During the period t t (Fig. 2) therefore, the anode andassociated shield are at approximately the same potential. The shieldsof the anodes notcarrying current are then at a potential different fromthat of their respective anodes owshield potentiallying below the anodepoteni tial. The maximum divergence occurs at time if, when the curve ais at Its maximum negative value and the difference ma is twice thedifierence between the voltages of Q and Fig. 3 illustrates thisarrangement diagrammatically. The same reference letters are used as inFig. 1. The difference lies in the fact of winding Q having more turnsthan winding Q so that the shields will be supplied with a highervoltage than the anodes, and also that the neutral points and 0 are notconnected. The potential of winding Q: is therefore entirely dependenton whichever anode is carrying load current at the time. Assume that anarc passes from anode a, through the shield 72., to the cathode K. Theshield h will then take up the same potential as the anode a Thepotentials of the other shields will then depend on the magnitude andsign of the potential drops existing between the shields. It will beeasily seen that in the case under consideration 1, 1 it follows thatthe shield It, must be negative with regard to the anode a It has beenassumed hitherto that the arc (load current) leaving the anode forms aconducting path between anode and shield and thus equalizes thepotentials of both. WVith shields of certain shapes or sizes it mayhappen, however, that the arc does not come into contact with the shieldat all, and its potential will therefore be indefinite. To ensure thatboth parts have the same potential the shields are given a certaindegree of activity by allowing them to work as anodes on to a loadingresistance. For this purpose a relatively large resistance is insertedbetween the neutral point 0 of the winding Q which supplies the shields,and the cathode. A small current then flows between the shields and thecathode, taking the form of an are which unites with the main are fromthe anodes. In this way the )resence of an electrically conducting pathetween anode and shield is ensured.

In applying the foregoing method for preventing baclcfires the effect ofvery small reverse currents on the shield potential must be taken intoconsideration. It must be assumed that a certain minimum reverse currentalways passing. The effect of this reverse current is to raise theshield voltage and. thus tends to counteract the externally impressednegative charge. There are various ways of rendering these reversecurrents ineffective. For example, a smaller control shield can befitted inside the anode shield proper and maintained at a potentialsomewhat lower than the anode. The outer shield then protects thecontrol shield to a certain extent from radiation from the arc and thusprevents its potential being raised by the reverse current. Another Waywould be to provide either the inner or the outer shields, or both, withan insulating coating which must also surround the lead to the shieldfrom the point where it enters the rectifier. This insulating coveringwould prevent reverse currents passing through the shields.

Finally, it is advisable to dispense entirely with all leads in theinterior of the rectifier since back-fires are very liable to start fromthe lead-in points. This may be done by adopting a known design in whichthe anodes are housed in tubular members of non-metallic insulatingmaterial which extend outside the rectifier. The shield is then placedround the lower part of the tubular member and connected directly to thecorresponding terminal of winding Q The resistances ee -w may then bedispensed with as no current can flow through the shields.

The anode shields may be replaced by other devices which when given apotential opposing the flow of current towards the anode eliminate thereverse currents. Equivalent devices may take the form of gratings,grids, rings etc.,1n short any insulated metal structure placed in thepath of the back-fire discharge.

What we claim is:

1. In an electrical system of the character described, the combinationwith a polyphase metal vapor rectifier comprising anodes and shieldsassociated respectively therewith, of means for impressing upon eachshield a potential negative with res ect to that on the respectiveassociated ano e during the entire non-working period of said anode.

2. In an electrical system of the character described, a polyphase metalvapor rectifier comprising anodes and shields associated respectivelytherewith, a polyphase winding supplying said anodes, and means forimpressing upon each shield a potential negative with respect to that onthe respective associated anode during a period of time immediatelyfollowing each normal operating period of said anode, said meansincluding a second polyphase winding.

3. In an electrical system of the character described, the combinationwith a polyphase metal vapor rectifier comprising anodes and shieldsassociated respectively therewith, of means for impressing upon eachanode and the associated shield similar sine waves of voltagecharacterized by the fact that at any instant of time the shield voltageis negative with respect to the anode voltage. I

4. In an electrical system of the character described, a polyphase vaporrectifier comprising anodes and shields associated respectivelytherewith, a polyphase winding supplying said anodes and having aneutral point, means for impressing upon each shield a potentialnegative with respect to that on the respective associated anode duringa period of time immediately following each normal operating period ofsaid anode, said means including a second polyphase winding having aneutral point, and a source of D. C. potential connected between saidneutral 10 points with the negative side of said source connected to theneutral point of said second polyphase winding.

In testimony whereof we have hereunto subscribed our names at Zurich,Switzerland 15 on the 18th day of May A. D. 1926.

JULIUS JONAS. ERWIN KERN.

